Natural teeth may be lost as a result of dental diseases or trauma making it desirable for replacement with prosthetic devices. One type of prosthetic device is a dental implant or root member, which is surgically positioned within the mandibular or maxillary alveolar bone. After healing a head member or abutment is mounted on the implant and then, in turn, a tooth simulating prosthesis, or crown, is mounted on the abutment.
Ravindernath and Mehta devised a dental implant comprising an implant base body for implantation in the predrilled whole in the jaw bone of the patient and a head member adapted to be interconnected with the said base body being provided for supporting prosthetic device for any tooth/teeth structure. The said implant base body has a flat top surface a conical and tapered lower end terminating in a flat base, a circular boundary and a parallel, non tapered sides extending from the top to the bottom. The said head member has a top surface and a bottom surface joined together through a straight parallel sides extending from the top to the bottom end of the shaft (INDIAN PATENT NUMBER 401/DEL/94). Another patent (INDIAN PATENT NUMBER 403/DEL/94) from the same inventors discloses a surgical dental implant, comprising a partly threaded cylindrical root member to be implanted in a predrilled bone of the jaw and to be interconnected with the head member, said root member has a flat top surface and a conical bottom end and said head member has a body part mounted on a shaft part. The drawbacks of the invention are its complete cylindrical shape which requires more bone removal and pressure during implant placement, crude thread design, non-self tapping threads, and no micro threads for crestal bone preservation.
U.S. Pat. No. 4,826,434 discloses a dental implant that can be constructed out of titanium so as to have a first end for insertion into a jawbone, an externally threaded shank extending from said first end, a tapered head on said shank adjacent to the second end of the implant and an internally threaded bore shape so as to include a non-round socket in the threads of said bore.
U.S. Pat. No. 5,061,181 discloses a dental implant anchor which includes a body portion having a first external wall portion carrying one or more circumferential projections separated by circumferential grooves and, below, a second external wall portion carrying threads. In some forms, the anchor also includes one or more longitudinally oriented grooves on its exterior wall. This invention does not have micro threads at the collar and the implant is partially threaded. Thus leading to limited crestal bone preservation and reduced primary stability for proper osseointegration of the implant with bone.
U.S. Pat. No. 5,064,425 discloses an elongate cylindrical body member adapted for permanent anchoring of its lower end into bone tissue and constructed to support an attached device such as a tooth at its upper end. The body member has an external screw thread formed in its outer surface, and at least one cavity defined therein adjacent to its bottom end. Each cavity forms at its intersection with the screw thread at least one cutting edge for self-tapping as the body member is screwed into the bone tissue. U.S. Pat. No. 5,312,256 discloses a dental implant which has a cylindrical body, with a rounded apical end. The cylindrical body has a succession of alternate flat parts and externally screw-threaded parts extending lengthwise of the body of the implant. There are two wide threaded parts for self-tapping and/or retention in already-tapped bone and two narrow threaded parts serving as guide rails during impacting and permitting the escape of chips during self-tapping. U.S. Pat. No. 5,727,943 discloses a self-tapping dental implant comprising a generally cylindrical body with a threaded outer surface. The cylindrical body has a plurality of longitudinal recesses formed in the threaded surface at one end and extends longitudinally through a plurality of turns of the thread to form a self-tapping cutting edge at each interruption of the thread by one of the recesses. The main drawbacks of the above inventions are their complete cylindrical shape which requires more bone removal during implant placement, limited self tapping nature of the threads, and no microthreads for crestal bone preservation.
U.S. Pat. No. 5,816,812 discloses a self-tapping dental prosthetic implant which has a blunt leading end, a tapered first section which has a uniform minor diameter and a uniformly increasing major diameter, a second section having uniform minor and major thread diameters, a third section with a uniform major thread diameter and a outwardly tapering minor diameter and a fourth section which has a diameter larger than any other segment and a relatively low profile (i.e., short axial length). A thread-cutting groove extends over a substantial portion of the threaded length of the implant.
U.S. Pat. No. 5,007,835 discloses a screw-typed dental implant which is featured having rounded threads for providing a controlled radial osteocompressive force against the threaded wall of bone tissue that was previously drilled and tapped. The thread profile provided by the tap is undercut below the external thread surface of the implant causing a compressive force to be exerted against the bone wall. The main drawbacks of the invention was the lower primary stability for proper osseointegration as the engagement with the bone was decreased due to the round shape of the threads and higher shear forces at the bone implant interface.
U.S. Pat. No. 5,074,790 discloses a screw implant for a jawbone that has a conical threaded implant body. The thread of the implant body is a compression thread with concave thread turns. The main drawback of the invention was the lower primary stability for proper osseo integration as the engagement with the bone was decreased due to the round shape of the threads. Higher shear forces at the bone implant interface.
U.S. Pat. No. 4,406,623 discloses a screw-type bone implant having a shaft with V-shape threads, particular length and the design of its free end, which helps its support between two ends by compact bone. Thus the implant offers a permanent and particularly secure support for a dental prosthesis. The disadvantages of the V-shape threads were development of high shear component of the axial load at the bone implant interface, which leads to faster loss of bone at the interface during function.
U.S. Pat. No. 5,823,777 discloses a dental implant having selected thread feature that could include the minor diameter, the thread pitch or the thread geometry (square threads). The selected biomedical characteristic is one of the locations in the bone in which the implant is placed, the elastic modulus of the bone and the desired biomechanical response of the bone. The disadvantages of the square thread design are the non-self tapping nature leading to more forces at the bone during insertion of the implant. This hampers the healing process after implant placement.
U.S. Pat. No. 5,601,429 discloses an anchor for a dental implant which preferably comprises of buttress type threads with a pitched relief groove extends across all convolutions of the external, self-tapping threads. The pitch of the relief groove is angled to vary from that of the threads, so that the groove intersects all threads, but the pitch is inclined in the same direction as that of the threads. The groove thus complements screwing action of the self-tapping thread. This invention utilizes the advantage of buttress threads however does not incorporate the microthreads at the collar which can reduce the crestal bone loss.
Reference may be made to U.S. Pat. No. 5,816,813 wherein an invention related to an implant having a body with at least one generally cylindrical part to be implanted into bone tissue is disclosed. The cylindrical part is at least partly provided with threads having a height between 0.02 mm and 0.20 mm. U.S. Pat. No. 6,419,491 discloses a dental implant system which includes an implant element for surgical insertion into a maxillofacial bone or tissue of a patient, the implant element having a collar section and a distal, anchor-like section, the collar section having an ordered microgeometric repetitive surface pattern in the form of alternating ridges and grooves, each having a fixed or established width in a range of about 2.0 to about 25 microns (micrometers) and a fixed or established depth in a range of about 2 to about 25 microns, in which the microgeometric repetitive patterns define a guide for preferential promotion of the rate, orientation and direction of growth colonies of cells of the maxillofacial bone or tissue which are in contact with the surface pattern.
U.S. Pat. No. 6,083,004 discloses an abutment-mount for a dental implant that has a longitudinally extending axis with a first end, an opposite second end and a peripheral surface. The abutment-mount is used for delivering the dental implant to a prepared site of a jawbone with an implant drive tool and is also used as a device for securing a dental prosthesis to the dental implant. The abutment-mount includes a screw, or other fastener for securing the abutment-mount to the implant. A surface is provided for attaching the dental prosthesis to the abutment-mount adjacent the first end. A structure is provided for transferring rotational force from the implant drive tool to the implant through the abutment-mount. An implant kit includes an abutment-mount, an implant and an abutment screw
U.S. Pat. No. 7,137,816 discloses that fixture mount limits torque that may be applied when installing a dental implant and also serves as an impression post and an abutment for a temporary prosthesis. In preferred embodiments, the fixture mount is formed of plastic and is subject to chair side modifications by the surgeon so as to be better suited as an abutment. Methods of shaping the fixture mount for use as an abutment and for making a dental restoration using the shaped abutment are disclosed.
Histologic and radiographic observations suggest that a biologic dimension of hard and soft tissues exists around dental implants and extends apically from the implant-abutment interface. Radiographic evidence of the development of the biologic dimension can be demonstrated by the vertical repositioning of crestal bone and the subsequent soft tissue attachment to the implant that occurs when an implant is uncovered and exposed to the oral environment and matching-diameter restorative components are attached. Historically, two-piece dental implant systems have been restored with prosthetic components that locate the interface between the implant and the attached component element at the outer edge of the implant platform. In 1991, Implant Innovations introduced wide-diameter implants with matching wide-diameter platforms. When introduced, however, matching-diameter prosthetic components were not available, and many of the early 5.0- and 6.0-mm-wide implants received “standard”-diameter (4.1-mm) healing abutments and were restored with “standard”-diameter (4.1-mm) prosthetic components. Long-term radiographic follow-up of these “platform-switched” restored wide-diameter dental implants has demonstrated a smaller than expected vertical change in the crestal bone height around these implants than is typically observed around implants restored conventionally with prosthetic components of matching diameters. This radiographic observation suggests that the resulting post restorative biologic process resulting in the loss of crestal bone height is altered when the outer edge of the implant-abutment interface is horizontally repositioned inwardly and away from the outer edge of the implant platform. This article introduces the concept of platform switching and provides a foundation for future development of the biologic understanding of the observed radiographic findings and clinical rationale for this technique (Lazzara and Porter, 2006).
Frost (1992) has proposed the hypothesis that bone cells respond to a local deformation of the bone produced by mechanical stress. The bone adapts to a certain strain—in a steady state. With slightly increased strain, the bone becomes mildly overloaded and compensates by forming more bone. If the strain goes beyond a threshold which exceeds the bone's capacity fatigue fracture can occur.
Typically a bone is believed to function within the strain range of approximately 50-1500 microstrain (Frost 2004). If the peak load on a bone results in strains of 1500-3000 microstrain a mild overload occurs. According to Frost's hypothesis (Frost 1992, 2004), this can result in mechanical fatigue damage, but remodeling normally repairs the damage and thus prevents it accumulating. Loads influencing the bone in this interval may even result in an osseous adaptation by formation of bone (reshaping and strengthening), presumably to reduce the future functional strain within the bone. Overloading the bone can increase the micro-damage (and the repair). Repeated stress on the bone resulting in deformations greater than 3000 microstrain increase the micro-damage. Such deformations can overwhelm the repair mechanism and result in a fatigue failure. In comparison, normal bone fractures suddenly at forces causing a deformation of about 2.5% (25,000 microstrain).
In contrast, if the strain in the bone does not exceed 50-100 microstrain, disuse of the bone occurs and remodeling results in a net loss of bone. Thus, a moderate increase from the optimal functional strains induces an increase in bone mass that, if the loading remains constant, re-establishes new optimal strains. Conversely, where functional loading is reduced to a level where optimal strains are not achieved, bone loss occurs to adapt to the new demand (Frost 1992).
It is important to appreciate that in this theory it is not the actual load that is important but the effect of the load on the bone—the resulting strain in the bone. This also depends on the amount of bone tissue. According to Frost (2004) a load of 1-2 MPa (approximately equivalent to 0.1-0.2 kg/mm2) results in 50-100 microstrain in cortical lamellar bone in healthy young adult mammals, and 60 MPa in 3000 microstrain. The level for sudden fracture is 25,000 microstrain and is obtained with a stress of 120 MPa. It has been suggested that there is not always a linear relationship between stress and bone failure, with one group reporting that a doubling of the stress that originally caused 2000 microstrain increased the microscopic fatigue damage in bone by 400 times (Pattin et al. 1996).
The stability of the dental implant system depends upon the biomechanical integrity of the components of the implant system and the support provided by the alveolar bone of the jaws. The biomechanical integrity of the components at the implant abutment interface and the alveolar bone level maintenance can be achieved by appropriate implant design. One of the aspects of implant design is the topography of the implant and the thread design.
Prior art has shown implant bodies mostly with cylindrical shape (Indian Patent No: 401/DEL/94 and 403/DEL/94; U.S. Pat. Nos. 5,312,256 and 5,727,943). However the cylindrical design has the disadvantages of causing excessive pressure on bone during insertion and unnecessary removal of bone for insertion. In the present invention the screw implants are made with the upper cylindrical portion and lower tapering portion to have the advantages of self-tapping, easy insertion in the bone, no excessive pressure at the implant bone junction, simulation of the natural anatomy of the root portion of the tooth, and avoiding injury to anatomical structures.
The implants with smooth and stepped topography does not obtain the desired postoperative primary stability, cannot maintain the alveolar bone level because of the stress shielding effect i.e. insufficient introduction of force to the bone for remodeling and have more shearing forces at the bone level under functional axial loading. The screw implants having external threaded surface have increased surface area which enhances osseointegrated bone implant contact level, have lower stress-shielding effect thus induces better remodeling of bone around implants, favorably transforms and distributes the shearing forces at the bone implant interface and better primary stability.
Thread shape is one of the most important factors which affects the forces applied on the bone and implant system during insertion and function. Prior art has revealed the use of different types of threaded designs like Standard V shape threads, Square shape threads, Buttress shape threads and Reverse Buttress threads (U.S. Pat. Nos. 4,406,623, 5,823,777 and 5,601,429). The comparison of these threads are made depending upon the—                1) location of stresses i.e. at the crest of alveolar bone and in the implant system under different type of loading forces,        2) transformation of axial loads into shearing component at the bone implant interface, and        3) Self-tapping nature.        
Standard V shape threads produce high shearing component of the axial load at the bone implant interface, thus leads to faster bone loss at the interface during function.
Square shape threads and reverse buttress threads have non-self tapping nature so produce increased forces during insertion at the bone implant interface, thus can lead to resorption of the bone affecting osseointegration process and stability of implant.
To overcome these limitations buttress threads are incorporated in the implant design to have self-tapping nature for easy insertion into the bone, transfer lesser functional forces at the implant abutment junction which is the most prone area for implant failure due to its lower strength, better primary stability, while producing moderate amount of forces at the alveolar crest during function under different type of forces and with comparable shearing component of the axial load at implant bone interface.